The field of ophthalmology for the past number of years has seen great strides in the development of refractive treatments intended to correct the vision of the eye. These techniques have evolved from the earlier radial keratotomy technique, in which slits in the cornea allowed the cornea to relax and reshape, to present techniques including photorefractive keratectomy (“PRK”), anterior lamellar keratectomy (“ALK”), laser in situ keratomileusis (“LASIK”), and thermal techniques such as laser thermal keratoplasty (“LTK”). All of these techniques strive to provide a relatively quick but lasting correction of vision.
With the development and refinements of these techniques, greater precision has become possible in refractive error correction. In early types of treatments, the precision of the correction was relatively coarse. To provide correction to within plus or minus one diopter of the desired correction for myopia, for example, would be considered an excellent outcome. The types of treatments have become progressively refined, however, allowing more subtle defects to be corrected. Myopia and hyperopia can now be corrected to a high degree of precision with current techniques, and using excimer lasers, higher order defects can also be corrected, such as asphericity and irregular astigmatism.
At the same time, the diagnostic tools to determine what correction is needed have also advanced. Employing topography systems, vision defects can be determined and corrected irrespective of their “regularity”. Such techniques are described in U.S. Pat. No. 5,891,132, entitled “Distributed Excimer Laser Surgery System,” issued Apr. 6, 1999. A variety of new topography systems, pachymetry systems, wavefront sensors, and overall refractive error detection systems can detect not only the amounts of myopia, hyperopia, and astigmatism, but also, higher order aberrations of the refractive properties of the eye.
Detection of wavefront aberrations in the human eye for such purposes as intraocular surgery and contact lens and intraocular lens fabrication is disclosed, e.g., in Liang et al, “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” Journal of the Optical Society of America, Vol. 11, No. 7, July, 1994, pp. 1–9. That technique will be summarized with reference to FIG. 1. A beam of light from a laser diode or other suitable light source is directed toward the pupil and is incident on the retina A beam (or wavefront, as described in FIG. 1) is reflected by the retina and emerges from the pupil. Typically, the incoming and emergent light follow a common optical path; the incoming light is brought into the common optical path with a beamsplitter. The emergent beam is applied to a Hartmann-Shack detector to detect the aberrations. Such a detector includes an array of lenslets which break up the light into an array of spots and focus the spots onto a charge-coupled detector (not shown in FIG. 1) or other two-dimensional light detector. Each spot is located to determine its displacement Δ from the position which it would occupy in the absence of wavefront aberrations, and the displacements of the spots allow reconstruction of the wavefront and thus detection of the aberrations through known mathematical techniques. In FIG. 1, θ is the locally averaged wavefront slope in front of the lenslet array and is related to the spot displacement and the lenslet focal length by θ=Δ/f, as will also be appreciated by those skilled in the art.
Improvements to the technique of Liang et al are taught in J. Liang and D. R. Williams, “Aberrations and retinal image quality of the normal human eye,” Journal of the Optical Society of America, Vol. 4, No. 11, November, 1997, pp. 2873–2883 and in U.S. Pat. No. 5,777,719 to Williams et al. (“Williams”). Williams teaches techniques for detecting aberrations and for using the aberrations thus detected for eye surgery and the fabrication of intraocular and contact lenses.
International Patent Publication WO 99/27334 (International App. PCT/UUS97/21688)(“Frey”) teaches a further variation using polarizing optics to control back-scatter from the lenses in the detector setup. Like Williams, Frey suggests using data from the wavefront sensor to develop an optical correction for the eye examined. More specifically, the optical correction so determined is limited to the aperture of the cornea measured by the sensor, e.g., the 6 millimeter circle to which the eye's pupil was dilated when the eye was measured. Outside that area, Frey suggests using a tapering blend zone of partial ablation to minimize severe changes in corneal curvature and hence lessen regression.
These diagnostic systems and techniques have the potential for permitting correction of both the fundamental and higher order defects, especially when used with the even more refined refractive correction techniques, with the possibility that vision correction to better than 20/20 will someday be the norm. However, improved techniques for applying advancing diagnostic technology to refractive surgery are needed.